1. Field of the Invention
The present invention relates to an electronic device capable of wired connection to an electronic device having a power-save mode.
2. Description of Related Art
A digital interface for digital AV (audiovisual) devices, HDMI (High-Definition Multimedia Interface) allows high-speed transfer of data such as of video and audio across a single cable. Modern digital AV devices, such as flat televisions and DVD recorders, are increasingly equipped with HDMI terminals as a standard feature.
Connection between digital AV devices having HDMI terminals is achieved via an HDMI cable, which includes a first to a fifth signal line described below.
The first signal line is a TMDS data signal line for one-way transfer—from source device to sink device—of audio and/or video data (hereinafter referred to as AV data) and info-frames (information on the format of AV data etc.) in a TMDS (Transmission Minimized Differential Signaling) format.
The second signal line is an HPD (Hot Plug Detect) signal line mainly used for indicating the timing of the start of transmission of AV data from source device to sink device.
The third signal line is a DDC (Display Data Channel) signal line used for transferring sink device-specific information (vendor name, model number, permitted resolutions, HDMI terminal numbers, etc.) to the source device. The DDC signal line is also used for authentication conforming to HDCP (High-bandwidth Digital Content Protection). When the HDMI cable is connected to the HDMI terminal of the sink device, the DDC signal line is connected to an NVRAM (non-volatile RAM) inside the sink device.
The fourth signal line is a CEC signal line used to achieve CEC (Consumer Electronics Control).
CEC is a capability that enables two-way control between source device and sink device, and is defined, along with AV data output specifications, in the HDMI standard. Nowadays, many CEC-ready electronic devices are commercially available. Since CEC allows two-way exchange of commands between source device and sink device, it permits both one-to-one and one-to-many control of devices.
The CEC standard defines a variety of functions. For example, with the function “One Touch Play,” when the “Play” button is pressed on the source device (e.g., a DVD recorder), the sink device (for example, a digital television) automatically shifts from stand-by mode, in which it remains in a power-saving, idle state, to power-on mode, in which a normal operation state is maintained, and selects as “input” the HDMI terminal connected via the HDMI cable to the source device. The CEC standard defines many other functions, and which functions to implement in products is up to manufacturers.
The fifth line is a DDC 5V signal line for supplying 5 V electric power from source device to sink device. The hardware configuration here is such that, after the sink device starts to be supplied with 5 V electric power via the fifth signal line from the source device, the sink device outputs a 5V HPD signal to an HPD pin of the HDMI terminal.
One way of using HDMI is to connect together, via an HDMI cable, a video camera having a power-save mode as a source device and a digital television as a sink device.
Generally, compared with stationary devices such as VCRs and DVD recorders, mobile devices such as video cameras and digital still cameras are designed with more emphasis placed on utmost reduction of electric power consumption. Accordingly, many mobile devices such as video cameras and digital still cameras are so configured that, if no user operation is detected for a predetermined period, they shift from power-on mode, in which they remain in a normally operating state, to power-save mode, in which they remain in a power-saving, idle state.
Typically, a video camera in power-save mode can be brought to normal operation mode by the user pressing the power switch or another key of the video camera, or operating the remote control dedicated to it. Instead, a CEC command requesting a shift to power-on mode (hereinafter referred to as a power-on request command) may be transmitted from the digital television to the digital video camera; this too brings the digital video camera in power-save mode to normal operation mode. At this time, however, depending on the state of the “input” currently being selected, the HPD signal may drop to low level. If the HPD signal drops to low level, the digital video camera may unexpectedly shift to power-save mode.
Likewise, when a power-on request command is transmitted from the digital video camera to the digital television, if switching of “input” occurs on the part of the digital television, the HPD signal may drop to low level. If the HPD signal drops to low level, the digital video camera may unexpectedly shift to power-save mode.